The LSE declined to give details on what had happened until the investigation was complete, and it is not known whether the new system was responsible. The system, written in C++ language and running on Novell SUSE Linux-based datacentres, replaced a Microsoft .Net-based system that was written in C# and ran on Windows Server and SQL Server.

The exchange said the closing auction started at 16:30:42 and not at 16:30:00 as scheduled. "Any securities executed with a UT [uncrossing trade] during the auction will remain valid. All ATs [automatic trades] that executed between 16:30:00 and 16:30:42 are under investigation," it said.

The opening day on the new system, yesterday, ran without any high profile problems, although customers of some data vendors outside the exchange said they experienced specific pricing problems with inaccurate data appearing. It has not been established whether the exchange’s technology was itself involved in the problems, though yesterday the LSE insisted its systems were running well.

The LSE said in a statement today that it “is currently investigating an issue affecting a number of securities and executions around the closing auction”. An update would be provided “as soon as possible”, it said.